Search for pulsations at high radio frequencies from accreting millisecond X-ray pulsars in quiescence

Abstract

Context. It is commonly believed that millisecond radio pulsars have been spun up by transfer of matter and angular momentum from a low-mass companion during an X-ray active mass transfer phase. A subclass of low-mass X-ray binaries is that of the accreting millisecond X-ray pulsars, transient systems that show periods of X-ray quiescence during which radio emission could switch on. Aims. The aim of this work is to search for millisecond pulsations from three accreting millisecond X-ray pulsars, XTE J1751−305, XTE J1814−338, and SAX J1808.4−3658, observed during their quiescent X-ray phases at high radio frequencies (5 ÷ 8 GHz) in order to overcome the problem of the free-free absorption due to the matter engulfing the system. A positive result would provide definite proof of the recycling model, providing the direct link between the progenitors and their evolutionary products. Methods. The data analysis methodology has been chosen on the basis of the precise knowledge of orbital and spin parameters from X-ray observations. It is subdivided in three steps: we corrected the time series for the effects of (I) the dispersion due to interstellar medium and (II) of the orbital motions, and finally (III) folded modulo the spin period to increase the signal-to-noise ratio. Results. No radio signal with spin and orbital characteristics matching those of the X-ray sources has been found in our search, down to very low flux density upper limits. Conclusions. We analysed several mechanisms that could have prevented the detection of the signal, concluding that the low luminosity of the sources and the geometric factor are the most likely reasons for this negative result.

Radio and X-ray monitoring of the accreting millisecond X-ray pulsar IGR J17591−2342 in outburst

“…However, we cannot rule out that a large eclipse duration or temporal broadening from scattering in the interstellar medium are responsible for the non-detection. Most AMXPs fail to show detectable radio pulsations during quiescence (e.g., Iacolina et al 2010;Patruno et al 2017), but this does not rule out that they become rotation-powered pulsars (e.g., Burderi et al 2003). Their rotation-powered activity during quiescence will be better probed by the future Square Kilometre Array (Keane et al 2015), which provides a major boost in sensitivity.…”

Radio Pulse Search and X-Ray Monitoring of SAX J1808.4−3658: What Causes Its Orbital Evolution?

“…No radio pulsations have been detected, setting the strongest possible upper limit (30 μJy at 2 GHz) on the presence of radio pulsations that exist (to date) for any AMXP. Iacolina et al (2010) performed radio pulsation searches toward SAX J1808.4-3658 at the higher observing frequency of 5 GHz and obtained an upper limit of 59 μJy on the presence of radio pulsations (this is equivalent to a 2 GHz upper limit of 280 μJy, if one assumes a power-law spectrum with index −1.7). Their approach assumed that detectability would be limited by freefree absorption, which could be avoided by observing at higher frequencies.…”

“…Hence, our observations were conducted at a relatively high central observing frequency of 2 GHz in orderto mitigate these effectswhile still maintaining sensitivity to the typically steep spectra of radio pulsars (ν −1.4 , where ν is the frequency of the electromagnetic radiation; see Bates et al 2013, although note that this result is specifictoisolated nonrecycled pulsars). A previousand complementarysearch of SAX J1808.4−3658 used an even higher observing frequency of 5 GHz in order to avoid free-free absorption from eclipsing material, but itwas correspondingly much less sensitive to steep-spectrum radio emission (Iacolina et al 2010).…”

“…There is also indirect observational evidence that SAX J1808.4−3658 turns on as a radio pulsar during quiescence, although no radio pulsations have been detected so far (Homer et al 2001;Burderi et al 2003;Campana et al 2004;Iacolina et al 2010). Indeed, the optical counterpart of SAX J1808.4−3658 is overluminous with respect to a nonirradiated brown-dwarf model (Bildsten & Chakrabarty 2001) during this phase.…”

SAX J1808.4−3658, an accreting millisecond pulsar shining in gamma rays?

“…For SAX J1808.4-3658, in particular, the amount of optical light reprocessed by the companion star during X-ray quiescence (Homer et al 2001) is compatible with irradiation by a radio pulsar (Burderi et al 2009), the decrease of the NS spin period between consecutive outbursts is similar to the rate observed from MSPs (Hartman et al 2009;Patruno et al 2012), and the rapid increase of the orbital period suggests ejection of the mass transferred from the companion star and/or changes in the mass quadrupole moment of the companion (di Salvo et al 2008;Burderi et al 2009;Patruno et al 2012). However, radio pulsations have not been detected from either SAX J1808.4-3658 or other AM-SPs (Burgay et al 2003;Iacolina et al 2009Iacolina et al , 2010, except for the case of IGR J18245-2453 (Papitto et al 2013). This could be due to an intrinsic low luminosity of the radio pulsar, geometrical effects, and/or free-free absorption from material ejected from the system by the pulsar radiation pressure (di Salvo et al 2008).…”